Method of grouting a drill hole

ABSTRACT

Calcium sulphate hemihydrate plaster slurry comprising a set-inhibiting agent, for example, a polymer or copolymer of an acid having the formula R 1  HC═CR 2  --COOH wherein R 1  and R 2  are hydrogen or methyl. The invention also includes a method of rapid setting of the inhibited slurry by mixing therewith a promoter comprising a salt of iron, copper or aluminium.

This is a division of application Ser. No. 826,190 filed Aug. 19, 1977,now U.S. Pat. No. 4,157,264.

This invention relates to a method of preparing hardened calciumsulphate hemihydrate plaster compositions, to set-inhibited(set-retarded) aqueous plaster slurry, to a set-promoting composition(hereinafter termed a promoter) to induce rapid-setting of said slurry,to a method of grouting a reinforcing dowel element in a drillhole in arock mass by means of said plaster composition and to a two-componentplaster grouting system wherein one component contains an inhibitedplaster slurry and the other contains said promoter.

There is increasing use of calcium sulphate hemihydrate plastercompositions for grouting reinforcing dowel elements in drillholes inrock masses as described in our co-pending United Kingdom patentapplication No. 45925/75. Such plasters are replacing polyester resingrouting composition for grouting anchor bolts and dowels in mines wherethe high flammability and toxicity of some of the ingredients of theresin compositions gives rise to an unacceptable risk. However, becausethe plaster sets within a short time of mixing with gauging water, theunset plaster slurry cannot be stored and must therefore be prepared onsite and injected into the drillholes shortly after mixing. The dowelsmust therefore be inserted into position in the drillhole before, orvery shortly after the plaster and there is always the risk of theplaster setting prematurely in the drillhole or in the injectionequipment. Since the method requires mixing and pumping equipment at thesite it is only practically applicable for large scale operations and itis inconvenient and wasteful when only a small number of fixing elementsare required to be secured at one time.

The setting period of calcium sulphate hemihydrate plasters may beextended by the inclusion of retarders, for example, protein, sodium andammonium salts of citric, gluconic and aconitic acids, and sodiumcarboxymethyl cellulose. The extension of setting time which can beachieved is, however, limited to hours or, at most, a few days and thetime of the eventual setting cannot be accurately controlled. Plastershaving long delayed setting times have not therefore been used asalternative to plasters mixed at the site although retarding agents arecommonly used to ensure adequate setting delay for preparing the plasterslurry and injecting it into the drillholes.

The polyester resin grouting compositions are more convenient thanplasters in many applications as the resins are normally used astwo-component systems, the components being brought together and mixedin the drillhole to prepare the setting composition. Thus resincompositions are available in which the separate components areencapsulated in frangible containers which are inserted into thedrillhole before the dowel, the containers being subsequently broken andthe contents mixed by rotating the dowel as it is inserted, as describedin United Kingdom patent specification Nos. 953,056 and 1,297,554.Separate encapsulation of the plaster and water component of plasterslurry has not proved practicable since the degree of mixing obtainablein the drillhole is not sufficient to give a grout of uniform strength.It would therefore be advantageous to provide an aqueous plaster slurrywhich could be stored without setting for a long period and be actuatedto set in a drillhole in a controlled manner when required. Accordinglyit is an object of this invention to provide such a plaster slurrycomposition.

We have now discovered that the setting of calcium sulphate hemihydrateplaster slurry can be delayed for long periods by the addition ofcertain inhibitors and we have further discovered that the set-delayingaction of these inhibitors can be neutralised by the addition of certaininorganic salts (herein termed promoters) to give extremely rapidsetting compositions.

In accordance with this invention a method of preparing a hardenedcalcium sulphate hemihydrate plaster comprises forming an inhibitedslurry plaster composition containing calcium sulphate hemihydrate,water and a set-inhibiting agent, mixing with said slurry a promoterreactable with the said set-inhibiting agent whereby the set-preventingeffect of the set-inhibiting agent is neutralised and setting of theslurry composition is induced.

A preferred inhibited calcium sulphate hemihydrate plaster slurry, whichslurry is also included in the invention, comprises calcium sulphatehemihydrate gauged with water containing in solution, as set-inhibitingagent, a polymer or copolymer of a monocarboxylic acid having theformula R¹ HC═CR² --COOH wherein R¹ and R² may be the same or differentand are hydrogen or methyl, or a water-soluble salt of said polymer orcopolymer. This inhibited plaster slurry is useful not only as aprecursor of a set plaster but, because of its enhanced thixotropy andlubricating properties, it is also an extremely effectively drilling aidfor oil well drilling.

Copolymers of acrylic acid or methacrylic acid and an alkyl ester ofacrylic acid or methacrylic acid containing 5 to 60 mole % of esterwherein the alkyl group contains from 1 to 8 carbon atoms, for example,methyl acrylate, have been found to be advantageous set-inhibitingagents. The polymeric set-inhibiting agent may conveniently be in theform of the sodium or ammonium salt. Preferably the molecular weight ofthe polymer or copolymer is within the range of 2,000 to 75,000. Whenthe gauging water, which generally constitutes about 30 to 40% w/w ofthe plaster slurry contains more than 0.1 part of polymericset-inhibiting agent to 100 parts of hemihydrate the setting is delayedalmost indefinitely. Preferably the concentration of the polymericset-inhibiting agent in the plaster slurry is in the range 0.4 to 1.0%by weight.

The inhibiting effect is most pronounced with plasters based on α-gypsumor β-gypsum and is best with the former grade of calcium sulphatehemihydrate.

In order to prevent water separating from the plaster slurry on storage,the slurry advantageously contains a macromolecular thickening agent,for example, water-soluble non-ionic cellulose ether or natural gum.Suitable thickeners include guar gum and the water-soluble alkyl,hydroxyalkyl, and alkyl hydroxy alkyl celluloses wherein the alkyl groupis methyl, ethyl or propyl. The viscosity of the thickener shouldpreferably be such that a 2% w/w solution in water at 20° C. has aviscosity of 20 to 100,000 centipoises, and more preferably theviscosity should be 3,000 to 7,000 centipoises. The concentration of thethickener in the plaster slurry is preferably 0.5 to 4% by weight.

The plaster slurry preferably also contains an antifoam agent tosuppress air entrainment and thickening of the plaster slurry duringsubsequent mixing operations. For this purpose the inclusion of 20 to200 parts per million of a silicone emulsion containing 10 to 30% ofdimethyl polysiloxane having a molecular weight of about 25,000 to30,000 is generally effective.

The plaster slurry may also optionally contain a filler material suchas, for example, china clay, perlite, fuel ash, quartz flour, titaniumdioxide or red iron oxide.

In preparing the plaster slurry the order of incorporation is notcritical but when the composition includes a thickener, the thickenershould preferably be first dissolved in the gauging water and theremaining ingredients then added to the solution.

The setting of the preferred inhibited plaster slurry containingpolymeric carboxylic acid may be rapidly effected in a controlled mannerwhen required by mixing with the slurry a promoter comprising at leastone iron, copper or aluminium salt. Aluminium salts are generallypreferable because of their lower toxicity. Ferric and cupric salts aremuch more effective than ferrous and cuprous salts and are thereforepreferable. The anion of the promoter salt is not critical, butnitrates, sulphates and chlorides are convenient.

Since excess promoter compositions can weaken the set plaster and thesetting time is not very dependent on promoter concentration, it ispreferable to employ the minimum quantity of promoter which can beadequately dispersed. Generally an amount of promoter providing from 0.5to 3 parts by weight of the active salt to 100 parts by weight ofplaster slurry is sufficient, higher concentrations being required forcupric and aluminium salts than for ferric salts.

The promoter is preferably employed as 1 to 25% w/w, preferably 5 to 20%w/w aqueous solutions and for convenient handling it is usuallyadvantageous to mix thickener and extending filler in the promotercomposition. Convenient thickeners include the water-soluble thickeningagents usable for thickening the plaster slurry, the preferredconcentration in the water being 0.1 to 10% w/w. Convenient fillersinclude china clay, quartz flour, titanium dioxide, red iron oxide,perlite and fuel ash, the amount of filler being preferably 50 to 500%by weight of the water. Although in preparing the thickened promotercomposition the order of mixing the ingredients is not critical, it isgenerally best to dissolve the thickener in the water before adding theremaining ingredients.

When promoter comprising ferric, cupric or aluminium salt is mixed withthe inhibited plaster slurry the mixture becomes highly thixotropicbefore hardening commences, the effect being more noticeable with ferricand cupric than with aluminium salts. This is an advantage when anon-flowing composition is desired, for example, for filling upwardlyinclined cavities. However when the mixing is being performed inrelatively inaccessible situations, for example, by means of a rotatingdowel in a drillhole, the initial stiffening makes mixing difficultbecause the dowel will tend merely to bore a hole in the thickenedplaster slurry. We have found that this initial stiffening can be muchreduced by partially reacting the iron, copper or aluminium salt of thepromoter with the aforedescribed polymeric carboxylic acidset-inhibiting agent, optionally in acid or salt form before mixing withthe inhibited slurry composition.

Preferably the promoter salt is reacted with 0.5 to 10 parts ofpolymeric carboxylic acid per 100 parts of salt. This partially reactedpromoter composition is easier to prepare than the unreacted promotersince, when the solid salt is added directly to an aqueous slurrycontaining thickener, polymeric carboxylic acid and filler, the salt isimmediately absorbed in the slurry to produce a thickened paste ofuniform consistency. The reacted promoter is thicker than correspondingunreacted formulations so less filler is necessary to obtain any givenconsistency and will not usually exceed the weight of the water. Thereacted promoter formulations are more easily extruded which is anadvantage for packing them into containers. Since the partially reactedpromoter is more easily and rapidly absorbed the promoter compositioncan contain a higher concentration of active salt, conveniently up to50% w/w in the aqueous solution. In addition the partially reactedpromoter reacts faster with polymeric carboxylic acid set-inhibitingagent in the plaster slurry and gives significantly reduced plastersetting times.

The controlled rapid setting of the plaster is extremely advantageous ingrouting dowels in rock and from a further aspect the invention consistsin a method of grouting a reinforcing dowel element in a drillhole in arock mass wherein the aforedescribed inhibited calcium sulphatehemihydrate plaster slurry mixed with a promoter comprising at least oneferric, cupric or aluminium salt is located around a dowel element in adrillhole and allowed to set.

The grouting materials are conveniently provided as two-components whichare provided separately and mixed within the drillhole and the inventionaccordingly includes a two-component grouting system wherein onecomponent contains the inhibited plaster slurry composition and theother contains the promoter.

The promoter is conveniently in a fluent form and both components may beconveniently injected simultaneously into the drillhole. Alternativelythe separate components may be encapsulated in frangible containers foreasy loading into the drillhole, the containers being adapted to bebroken and the contents mixed by a dowel which is rotated as it isinserted into the drillhole. The containers may be conveniently madefrom any waterproof film material and containers of thermoplastics, forexample, polyethylene, polypropylene, polyethylene terephthalate andpolyvinyl chloride are particularly suitable.

In addition to its advantages in the preparation of grouting materialthe rapid plaster hardening system of the invention is advantageous inother applications where an easily prepared rapid setting plaster isrequired. Thus the system can be used in the preparation of wallplasters from permanently inhibited wet slurry compositions. Also, inconditons of emergency involving leakage of toxic or flammable gases orliquids, such as can occur in oil and natural gas wells and in coalmines, the system may be used to provide a barrier to prevent theleakage or the spread of flames, or to prevent air reaching thepotentially dangerous substance.

The invention is further illustrated by the following Examples whereinall parts and percentages are by weight.

In these Examples the calcium sulphate hemihydrate was α-gypsum andβ-gypsum supplied by British Gypsum Limited as Cristocal and Herculite(Registered Trade Marks) respectively. 98% of the gypsum particles werebetween 0.5 and 1.5μ.

The china clay particle size was 25% <2μ, 66% 2-10μ and 9% >10μ.

The thickener was hydroxypropyl methyl cellulose having a molecularweight of 156,000, a degree of substitution of 1.5 for methyl and 0.35for hydroxypropyl and the viscosity of a 2% aqueous solution was 4,500cps at 20° C.

The choice of thickener was in no way critical and similar results wereobtained using methyl cellulose, hydroxyethyl cellulose, methylhydroxyethyl cellulose, ethyl hydroxyethyl cellulose, hydroxypropylcellulose, methyl ethyl cellulose, hydroxypropyl methyl cellulose, guargum and oxidised guar gum with widely varying viscosity and molecularweight.

The silicone antifoam was a 30% w/w dispersion of polydimethylsiloxane,molecular weight 25,000-30,000 in water.

EXAMPLE 1 Preparation of Plaster Slurry

90 g of hydroxypropyl methyl cellulose were dissolved in 8.73 kg ofwater (gauging water) and 1.0 g of silicone antifoam and 180 g of a 40%w/w aqueous solution of a sodium salt of a copolymer of acrylic acid andmethyl acrylate containing 50 mole % of methyl acrylate and having amolecular weight of 3,000 to 5,000 were added and completely dispersedin the solution in a dough mixer with a vertical rotating blade stirrer.23.1 kg of α-gypsum were added to the thus prepared gauging solution inthree approximately equal portions and the resulting slurry was mixedfor about 5 minutes until it was uniform. Excessive mixing was avoidedas it can cause undesirable air entrainment. The bulk density of theslurry was 1.79 gm/cc and the viscosity as measured by a Brookfield RVTviscometer was

    ______________________________________                                        Shear Rate (RPM)                                                                            Spindle No   Viscosity (cps)                                    ______________________________________                                        0.5           6            250,000                                            100           6             7,700                                             ______________________________________                                    

Preparation of Promoter Slurry

50 g of hydroxypropyl methyl cellulose were dissolved in 5.0 kg of waterand 200 g of the 40% w/w solution of acrylic copolymer, as used in thepreparation of the plaster slurry, were mixed into the solution in adough mixer with a vertical rotating blade stirrer. 5.0 kg of china claywere added in three approximately equal portions and mixed until theslurry was uniform. 2 kg of aluminium sulphate hydrate Al₂ (SO₄)₃.14H₂ Owere added in powder form and mixing continued until a uniform paste wasproduced. The temperature of the slurry increased to a maximum of 38° C.due to the reaction of the inorganic salt with the acrylic polymer.

The viscosity of the promoter slurry at 20° C. as measured by aBrookfield RVT model viscometer was

    ______________________________________                                        Shear Rate (RPM)                                                                            Spindle No   Viscosity (cps)                                    ______________________________________                                        0.5           6            700,000                                            100           7             6,400                                             ______________________________________                                    

Samples of the plaster slurry were mixed with varying amounts of thepromoter slurry and the setting times were as shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Parts of promoter slurry/                                                                    2.5    5.0    10.0 15.0 20.0 40.0                              100 parts plaster slurry                                                      Set time                                                                      (in minutes)   75.0   4.25   4.25 4.0  4.0  3.5                               ______________________________________                                    

Tensile strength, measured as the breaking load of a dumb-bell shapedtest column having a cross-sectional area of 6.3 cm² made from theplaster slurry containing 10 parts of promoter slurry/100 parts ofplaster slurry was 1,550 kilo-newtons/meter².

Anchorage strength, measured by pulling a 25 mm diameter ribbed steeltest bolt set into a 35 mm×150 mm hole in matured portland cementconcrete (compressive strength 27,500 kilo-newtons/meter²), with theslurry containing 10 parts of promoter slurry/100 parts of the plasterslurry was 3.2 kilo-newtons/cm at 24 hours after setting, which is aboutthe same as the strength obtained with unretarded plaster.

EXAMPLE 2

A plaster slurry was prepared by the procedure described in Example 1(using the gauging solution) but in this Example 18.0 kg of α-gypsumwere added to the gauging solution in two approximately equal portionsfollowed by 5.1 kg of fuel ash and the slurry was mixed until it wasuniform. The bulk density of the slurry was 1.01 gm/cc and the viscosityas measured by a Brookfield RVT viscometer was

    ______________________________________                                        Shear Rate (RPM)                                                                            Spindle No   Viscosity (cps)                                    ______________________________________                                        0.5           6            650,000                                            100           7             12,000                                            ______________________________________                                    

The fuel ash was an aluminium silicate with the following analysis

    ______________________________________                                        Silica (as SiO.sub.2)  55-60%                                                 Alumina (as Al.sub.2 O.sub.3)                                                                        25-30%                                                 Iron (as Fe.sub.2 O.sub.3)                                                                            4-10%                                                 Ca (as CaO)            0.2-0.6%                                               Mg (as MgO)             1-2%                                                  Alkalis (as Na.sub.2 O,K.sub.2 O)                                                                    0.5-4%                                                 ______________________________________                                    

The particle size was 10-250 microns, approximately 5% being below 50microns and 20% being above 125 microns.

100 parts of the plaster slurry were mixed with 10 parts of the promoterslurry as prepared in Example 1 and allowed to harden. The setting timewas 4.5 minutes. The tensile strength and achorage strength of the setplaster measured as described in Example 1, were 950 kilo-newtons/meter²and 1.8 kilo-newtons/cm respectively at 24 hours after setting.

EXAMPLE 3

10 parts of the plaster slurry and 1 part of promoter slurry as preparedin Example 1 were pumped simultaneously into a 1.8 meter×42 mm diameterdrillhole inclined upwardly at about 45° from horizontal in anunderground coalface, using a dual feed metering pump. After theslurries had almost filled the hole, the feed lines were removed and a1.8 meter×30 mm diameter wood dowel was inserted into the drillhole androtated in the hole at about 120 RPM for 1 minute. The wooden dowel wasfirmly secured in the drillhole after about a further 2 minutes.

EXAMPLE 4

The promoter composition as prepared in Example 1 was extruded into apolyethylene terephthalate tubular film of 20 mm diameter and 0.037 mmwall thickness at a loading of 150 g/meter length.

The promoter-containing tube was placed inside a second tubularcontainer made from polyethylene terephthalate film of 40 mm diameterand 0.025 mm wall thickness and this outer container was filled withplaster slurry as prepared in Example 1 by extrusion. The continuouslength of the filled tubular container was divided into 33 cm lengthswhich were crimped and sealed at both ends to encapsulate the plasterand promoter. Each capsule contained approximately 570 g of plasterslurry and 50 g of promoter slurry.

These capsules were stored at between 20° C. and 30° C. for 12 weekswithout any sign of setting and only a trace of water separation.

Viscosities of the plaster and promoter slurries after 12 weeks were

    ______________________________________                                                   Plaster Slurry                                                                            Promoter Slurry                                        ______________________________________                                        0.5 RPM (spindle 6)                                                                        1,100,000     950,000                                            100 RPM (spindle 7)                                                                          36,000       11,200                                            ______________________________________                                    

Although there was some thickening of both the plaster and promoter,both remained in workable condition.

Four capsules as produced in this Example were placed in a 1.8 meter×42mm diameter drillhole inclined upwardly at about 45° from horizontal inan underground coalface. A 1.8 meter×30 mm wood dowel was rotated atabout 120 RPM while it was being pushed to the back of the hole. Theoperation required about 45 seconds, and the dowel was rotated for afurther 15 seconds. After about 3 minutes the dowel was set solid in thedrillhole.

EXAMPLE 5

A plaster slurry was made using the same ingredients and proportions asin Example 1 except that β-gypsum was used instead of α-gypsum. Althoughthe initial viscosity was not much different from material of Example 1,the plaster slurry thickened considerably after 12 weeks storage and itwas not possible to measure its viscosity.

Tensile strength and anchorage strength of 100 parts of the freshplaster slurry reacted with 10 parts of the promoter slurry as used inExample 1 was 1,400 kilo-newtons/metro² and 2.4 kilo-newtons/cm oflinear bond respectively after 24 hours setting.

EXAMPLE 6

Several promoter slurries were prepared as described in Example 1varying the inorganic salt and its concentration and these were mixedwith plaster slurry as prepared in Example 1. The effect on the settingtime of the type of inorganic salt, concentration of salt and amount ofthe promoter slurry used is shown in Table 2.

                                      TABLE 2                                     __________________________________________________________________________    Parts inorganic                                                               salt/100 parts                                                                          5                  10                                               water in promoter                                                             slurry                                                                        Parts of promoter                                                             slurry mixed with                                                                       2.5                                                                              5.0                                                                              10.0                                                                              15.0                                                                             20.0                                                                             40.0                                                                             2.5                                                                              5.0                                                                              10.0                                                                             15.0                                                                             20.0                                                                             40.0                              100 parts of                                                                  plaster slurry                                                                Promoter inorganic                                                            salt      set time (minutes)                                                  Fe Cl.sub.3                                                                             NS 60.0                                                                             3.0 2.33                                                                             2.25                                                                             2.25                                                                             60.0                                                                             5.5                                                                              5.0                                                                              5.0                                                                              5.5                                                                              4.0                               CuSO.sub.4 . 5H.sub.2 O                                                                 NS NS 60.0                                                                              6.0                                                                              4.5                                                                              4.33                                                                             NS 7.25                                                                             6.0                                                                              4.75                                                                             4.33                                                                             4.0                               Al.sub.2 (SO.sub.4).sub.3 . 14H.sub.2 O                                                 NS NS 180.0                                                                             10.0                                                                             8.0                                                                              5.25                                                                             NS NS 8.0                                                                              6.25                                                                             4.75                                                                             4.5                               Parts inorganic                                                               salt/100 parts                                                                          15                 20                                               water in promoter                                                             slurry                                                                        Parts of promoter                                                             slurry mixed with                                                                       2.5                                                                              5.0                                                                              10.0                                                                              15.0                                                                             20.0                                                                             40.0                                                                             1.0                                                                              2.5                                                                              5.0                                                                              10.0                                                                             15.0                                                                             20.0                              100 parts of                                                                  plaster slurry                                                                Promoter inorganic                                                            salt      set time (minutes)                                                  Fe Cl.sub.3                                                                             6.0                                                                              5.0                                                                              4.0 3.5                                                                              3.5                                                                              4.0                                                                              60.0                                                                             5.5                                                                              4.0                                                                              4.0                                                                              4.5                                                                              4.5                               CuSO.sub.4 . 5H.sub.2 O                                                                 -- 5.5                                                                              3.5 3.25                                                                             3.25                                                                             3.0                                                                              -- 6.5                                                                              3.5                                                                              2.75                                                                             2.5                                                                              2.75                              Al.sub.2 (SO.sub.4).sub.3 . 14H.sub.2 O                                                 NS 9.5                                                                              5.5 4.5                                                                              4.5                                                                              4.0                                                                              NS NS 6.5                                                                              4.0                                                                              4.25                                                                             4.25                              __________________________________________________________________________     NS = No Set                                                              

EXAMPLE 7 PREPARATION OF PROMOTER SLURRY

50 g of hydroxypropyl methyl cellulose were dissolved in 5 kg of waterand 500 g of ferric chloride were dissolved in the solution in a doughmixer. 7.5 kg of china clay were mixed into the solution with stirringuntil a uniform slurry was obtained. The viscosity of the promoterslurry at 20° C. measured by a Brookfield RVT model viscometer was

    ______________________________________                                        Shear Rate (RPM)                                                                            Spindle No   Viscosity (cps)                                    ______________________________________                                        0.5           6            410,000                                            100           7             12,000                                            ______________________________________                                    

Samples of the promoter slurry were mixed with plaster slurry asprepared in Example 1 and the variation of the setting time of theplaster with varying amounts of promoter slurry was as shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        Parts of promoter slurry/                                                                    5.0    10.0   20.0 40.0 80.0 120.0                             100 parts plaster slurry                                                      Set time       10.0   10.25  12.25                                                                              15.25                                                                              17.0  24.0                             (in minutes)                                                                  ______________________________________                                    

The tensile strength and anchorage strength of set plaster made from 100parts of the slurry and 5 parts of the promoter slurry measured asdescribed in Example 1 were 1,750 kilonewtons/meter² and 4kilo-newtons/cm respectively.

EXAMPLE 8

Several promoter slurries were prepared as described in Example 7varying the inorganic salt and its concentration and the promoter slurrywas mixed with plaster slurry as prepared in Example 1. The variation insetting time with type of inorganic salt, concentration of salt andamount of promoter slurry used is shown in Table 4.

                                      TABLE 4                                     __________________________________________________________________________    Parts inorganic                                                               salt/100 parts                                                                          5           10               15                                     water in                                                                      promoter slurry                                                               Parts of promoter                                                             slurry mixed with                                                                       5.0                                                                              10.0                                                                             20.0                                                                             40.0                                                                             2.0                                                                              5.0 10.0                                                                              20.0                                                                             40.0                                                                             2  5  10 20 40                         100 parts of                                                                  plaster slurry                                                                Promoter inorganic                                                            salt      Set time (minutes)                                                  Fe Cl.sub.3                                                                             26.0                                                                             8.0                                                                              10.0                                                                             10.5                                                                             -- 10.0                                                                              10.25                                                                             12.5                                                                             15.25                                                                             11.25                                                                           10.5                                                                             10.0                                                                             11.5                                                                             12.0                       Fe.sub.2 (SO.sub.4).sub.3                                                               NS ON 10.0                                                                             14.0                                                                             -- 11.5                                                                              9.5 10.5                                                                             10.5                                                                             12.0                                                                             11.25                                                                            10.25                                                                            11.33                                                                            12.25                      Cu SO.sub.4 . 5H.sub.2 O                                                                -- -- -- -- ON 11.0                                                                              7.0 5.5                                                                              6.5                                                                              ON 18.0                                                                             7.5                                                                              6.25                                                                             5.0                        Al.sub.2 (SO.sub.4).sub.3 . 14H.sub.2 O                                                 -- -- -- -- NS 200 100 10.5                                                                             9.0                                                                              -- -- -- -- --                         AlCl.sub.3 . 6H.sub.2 O                                                                 -- -- -- -- NS NS  15.0                                                                              13.0                                                                             13.0                                                                             -- -- -- -- --                         __________________________________________________________________________     NS = No set                                                                   ON = Overnight set                                                       

EXAMPLE 9

Several plaster slurries using various grades of polyacrylic acids andtheir salts were prepared as described in Example 1 in order to testtheir effectiveness as retarders for α- and β-gypsum plasters.

Samples were stored in sealed jars at 50° C. for 1 week and examined forsigns of solidification. The condition of the samples is given in Table5.

After storage the samples were mixed with promoter slurry as describedin Example 1 and all the plasters set in a satisfactory manner.

                  TABLE 5                                                         ______________________________________                                                                            Conditions                                      Polyacrylic Molecular Plaster after 1 week                              Sample                                                                              acid type   Weight    Type    at 50° C.                          ______________________________________                                        A     PAA         3,500     α-gypsum                                                                        No thickening                                                                 or gelation                               B     PAA         3,500     β-gypsum                                                                         Some thick-                                                                   ening, no                                                                     gelation                                  C     PAA         27,000    α-gypsum                                                                        No thickening                                                                 or gelation                               D     PAA         76,000    α-gypsum                                                                        Some gelling                              E     PAA         5.1 × 10.sup.6                                                                    α-gypsum                                                                        Gelled                                    F     Sodium      4,000     α-gypsum                                                                        No thickening                                   salt PAA                      or gelation                               G     Sodium      4,000     β-gypsum                                                                         Thickened                                       salt PAA                      but no                                                                        gelation                                  H     Ammonium    4,000     α-gypsum                                                                        No thickening                                   salt PAA                      or gelation                               J     Ammonium    4,000     β-gypsum                                                                         Thickened                                       Salt PAA                      but no                                                                        gelation                                  K     Sodium      4,000     α-gypsum                                                                        No gelation                                     salt AA/MA                                                                    copolymer (50                                                                 mole % AA)                                                              L     Sodium      4,000     β-gypsum                                                                         Thickened                                       salt AA/MA  4,000     β-gypsum                                                                         but no                                          copolymer (50                 gelation                                        mole % AA)                                                              ______________________________________                                         PAA = Polyacrylic acid                                                        AA = Acrylic acid                                                             MA = Methyl acrylate                                                     

EXAMPLE 10

This Example illustrates the use of a composition of this invention forfilling vertical cracks.

A plaster slurry as prepared in Example 1, was mixed by hand stirring ina disposable paper cup, with about 10% of its weight of a promoterslurry as prepared in Example 6. The colour of ferric chloride in thepromoter slurry was used as a visual indicator for the dispersion of thepromoter slurry in the plaster slurry. A thixotropic mix having auniform light brown colour was obtained in about 1 minute of mixing. Themix was trowelled into a 23 cm×4 cm×2.5 cm deep crack in a verticalsandstone wall, and worked to an even finish. The plaster set solidafter about 8 minutes and was painted over to match the colour of thesandstone wall immediately thereafter.

What I claim is:
 1. A method of grouting a drillhole with a hardenedcalcium sulphate hemihydrate plaster comprising forming a set-inhibitedslurry composition containing calcium sulphate hemihydrate, water anddissolved set-inhibiting agent, said set-inhibiting agent being selectedfrom the group consisting of water-soluble polymers and copolymers ofacrylic acid, and water-soluble salts of said polymers and copolymers,said polymers and copolymers having molecular weights within the range2,000 to 75,000 and mixing with said slurry in a drillhole a promoterreactable with said set-inhibiting agent, said promoter comprising atleast one salt selected from the group consisting of the water-solublesalts of iron, copper and aluminium, whereby the effect of theset-inhibiting agent is neutralized and the setting of the compositionis induced.
 2. A method as in claim 1 wherein the set-inhibited slurryplaster composition and the promoter are provided in separate componentsof a two-component grouting system.
 3. A method as in claim 2 whereinboth components are injected simultaneously into the drillhole.
 4. Amethod as in claim 2 wherein the separate components are encapsulated infrangible containers which are broken and the contents mixed by a dowelinserted into the drillhole.
 5. A method as in claim 2 wherein theslurry composition and promoter are located around a dowel element in adrillhole and are allowed to set.
 6. A method as in claim 5 wherein theplaster slurry composition and the promoter are provided as separatecomponents of a two-component system and mixed within the drillhole. 7.A method as in claim 6 wherein the separate components are injectedsimultaneously into the drillhole.
 8. A method as in claim 6 wherein theseparate components are encapsulated in frangible containers, thecontainers are loaded into the drillhole and the dowel is subsequentlyinserted with rotation into the drillhole whereby the frangiblecontainers are broken and their contents mixed.